70 MORPHOLOGY AND EVOLUTION Hoolock Gibbons as follows: "The voice of our Hainan G-ibbon is quite different from that of the Hoolock. It is a high-pitched trill all on the same note, and shriller even than the high note of the Hoolock's cry. It consists of about three to six distinct cries repeated in very rapid succession, suggesting almost production by vibration of the tongue, although, as a matter of fact, I believe the lips alone are instrumental in producing the effect. There is then a momentary pause, 'after which the cry is repeated. It may perhaps be represented in the following way: huo hoo hoo hoo—hoo hoo hoo—hoo hoo hoo hoo hoo, &c. The Hoolock, on the contrary, cries as follows: hah, hoo, hah, hno, hah, hah, huo, hah. The ' hdo ' is on a lower note than the ' hah,' with which the cry frequently ends. The ordinary expression of anger or remonstrance in the Hainan Gibbon is a prolonged and guttural grunt, repeated rapidly and often, and frequently interspersed with a kind of warble when the excitement rises." In captivity Gibbons dip the backs of their hands and knuckles in the drinking troughs, and lick the fluid off the hairs; but it is incorrect to say, as Tickell (215) has, * that some species scoop up the water. In their natural surroundings they can wipe rain-water off leaves, their long arms enabling them to reach much foliage ; or they can descend to the sides of lakes and employ the methods which they exhibit in captivity. If they are observed from a distance in the jungle the females, with young ones clinging to their breasts, engaged in the process of hand-drinking, may appear to be washing the faces of their offspring.